Pneumatic conveying systems have been used to transfer the contents of bulk containers into in process storage hoppers or bins for further processing. In situations where non-dusty materials are being handled, open systems or semi-closed systems have been employed. These open systems essentially use the air once through for conveying purposes and discharge the conveying air from the system. A vacuum type open system is illustrated in FIG. 2 of U.S. Pat. No. 3,809,438. A pressure type open system is illustrated in U.S. Pat. No. 4,284,372. In both such systems, air or another conveying medium is taken into the system at one point, used to convey the materials and is allowed to exit the system after the conveyed materials have been discharged into a receiving receptacle.
Closed loop systems have been developed for transferring materials from bulk storage containers into storage bins. These systems are necessary for bulk users of granular or powdered materials who obtain such materials by shipments in bulk containers such as railroad hopper cars or hopper trucks.
The hopper cars or trucks have been unloaded by connecting a vacuum source to a product outlet on the underside of the bulk transfer container. The discharge header of the hopper car or truck was connected to a centrifugal cyclone separator. The overheads from the cyclone separator were piped into an inlet of a fan or blower while the particulate discharge connection on the cyclone separator was connected into the storage container or bin for further processing. The discharge of the fan of blower was directed back to the discharge manifold of the railroad hopper car or truck.
These types of systems suffered from several disadvantages. For one, the use of vacuum to unload railroad hopper cars required relatively short distances between the hopper truck or car being unloaded and the in-process storage bin to which the contents would ultimately be transferred. Additionally, maintaining the proper pressure and vacuum zones within the circuit required use of in line devices which would tend to cause plugging problems with resulting system downtime. One of such closed circuit pneumatic conveying systems is U.S. Pat. No. 3,694,037, wherein the balancing problem between the pressure and vacuum zones within the circuit is illustrated. In U.S. Pat. No. 3,694,037, the fan discharge line required a pressure relief connection with a filter sock mounted to it as well as an inline restriction to control the transition point between pressure and vacuum.
Another shortcoming of the closed loop systems used in the past was that significant amounts of the material being conveyed would not be sufficiently separated by the cyclone and would thus have to be processed through the circulating fan. Certain conveying systems adopted specially designed fans with separate dust recirculation lines to attempt to minimize the amount of dust flowing through the system fan or blower. One such system is illustrated in U.S. Pat. No. 3,809,438 (FIG. 1).
In many transfer operations involving the unloading of bulk containers, it was frequently necessary for operators to employ the same facilities to unload a wide variety of materials into separate storage containers. In using a closed loop system for products of different types, it was necessary to be able to insure adequate cleaning of the entire system in between products so as to avoid cross-contamination. Accordingly, the system had to be kept as simple as possible and as short as possible to facilitate proper washing of all the ducts and equipment in between products. As a result, complex equipment such as filter baghouses had to be eliminated since such equipment was difficult to effectively clean. Despite the requirement of easy cleaning which precluded the use of complex filtration devices, it was also desirable to minimize product losses from the transfer system for environmental as well as cost reasons.
The apparatus and method of the present invention address the problem of cross-contamination by employing few system elements each of which is readily cleanable. Furthermore, the apparatus and method of the present invention provide significant containment within the system of the material being transferred, thereby minimizing product losses.